Parkinson's disease (PD) is a common, disabling, and incurable neurodegenerative disorder with evidence for substantial heritability. Approximately 30 distinct PD risk loci have been identified based on genome-wide association studies (GWAS). However, most of the implicated genomic regions contain multiple genes that are equally likely to be affected by associated polymorphisms, and potential functions for the majority of these gene candidates are completely unknown. Therefore, the critical next steps for translational research in PD genomics are (1) to determine the responsible gene(s) at each susceptibility locus, and (2) to define their impact on disease mechanisms. Answering these questions will be essential for turning insights from human genetics into clinical breakthroughs in PD therapeutics. Based on overwhelming evidence, alpha-synuclein (alphaSyn) neurotoxicity is a central mechanism in PD susceptibility and pathogenesis, and we hypothesize that many risk loci may modulate the relevant biological pathways. We have developed an improved model of synucleinopathy in the fruit fly, Drosophila melanogaster, that dynamically and robustly recapitulates alphaSyn toxicity in the adult nervous system, including early dysfunction in synaptic transmission and progressive neurodegenerative cell loss. Importantly, this system is amenable to high-throughput genetic screening. In Aim 1, we will leverage the rapid capabilities of fly genetics and the availability of near-saturation reagents for the Drosophila genome to comprehensively evaluate candidate genes at ~30 PD susceptibility loci from human GWAS. For ~100 genes, we will identify Drosophila orthologs, obtain lines predicted to disrupt or activate gene function, and screen for genetic interactions with alphaSyn neurotoxicity in vivo. A versatile and high-throughput screen in the fly retina will allow parallel assessments of alphaSyn-mediated structural and functional neurodegeneration. In Aim 2, the most promising modifier genes will be validated in the adult Drosophila brain examining age-dependent, alphaSyn-induced dopaminergic neuronal loss and resulting locomotor impairment. In order to define the key mechanisms, the top modifier genes demonstrating consistent and robust interactions will be profiled in vivo for their impact on alphaSyn dynamics, including protein levels, phosphorylation, misfolding/aggregation, and inclusion formation. The proposed strategy will enable us to move efficiently from lists of associated polymorphisms to in vivo confirmation of the causal genes and validation of roles in alphaSyn- mediated neurodegeneration. The definitive identification and functional elucidation of PD susceptibility genes holds great promise for the discovery of new treatment strategies for this devastating disorder.